Gain stabilizing circuit for electron multiplier phototubes



June 18, 1957 E. E. GOODALE 2,796,531

GAIN STAB ILIZING CIRCUIT FOR ELECTRON MULTIPLIER PHOTOTUBES Filed May 17, 1954 26 OUTPUT HIGH VOLTAGE l7 suPPLYfi m D \l VOLTS PE R STA GE I hventor: Edmund E. Goods/e,

balm His Attorney.

Patented June 18, 1957 ice GAIN STABILIZING IIRCUIT FOR ELECTRON RHILTIPLER PHOTOTUEES Edmund E. Goodale, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application May 17, 1954, Serial No. 430,185

5 Claims. (Cl. 25ll207) This invention relates to electron multiplier phototubes, and more particularly to an improved circuit for such phototubes whereby the overall amplification or gain of the phototube may be stabilized at substantially a constant level despite variations in applied operating voltage, and whereby the level at which the gain is stabilized may be adjusted.

Electron multiplier phototubes are well known in the art, and are employed in a variety of applications where higher amplification is required than can conveniently be obtained through the use of conventional vacuum tube amplifiers. Typical uses are in scintillation counters, in spectrometers and spectrographs, and in various instruments that are concerned with X-ray intensity measurements. Several types of multiplier phototubes have been devised, but the basic operation of all types depends on the phenomena of secondary emission to amplify electron streams. In the electrostatic type, with which the present invention is concerned, the phototube comprises a photosensitive electron-emittiag cathode, a series of intermediate increasing stream of electrons, until those emitted from the.

last dynodes are collected by the anode. Those electrons collected by the anode constitute the current utilized by.

the output circuit.

The potentials for the various electrodes generally obtained from a voltage divider that is connected across the power supply, and generally comprises a plurality of series-connected fixed resistors. It is apparent that if the output voltage of the power supply varies/the voltage drops across the resistances comprising the voltage divider will vary, and, consequently, the potentials on the various electrodes of the phototube will vary. It is well known that the amplification or gain of an electron multiplier phototube is directly related to the voltage dilference between adjacent electrodes (volts per stage) of the tube. The overall amplification of the phototube is equal to the average amplification per stage raised to the. nth power, where n is the number of stages, and thus small variations in the amplification per stage produce large changes in the overall tube amplification. Therefore, a small change in the level of the output voltage of the power supply, which causes a small change in the amplification of each stage of the tube, will cause the overall gain of the tube to vary between wide limits. This, of course, prevents the use of electron multiplier phototubes in many instruments where their use would be desirable, but where the provision of a large, well-regulated power supply is imof the tube are practical. Therefore, a primary object of the present invention is to provide a circuit for an electron multiplier phototube that permits the overall gain of the tube to be stabilized at substantially a constant level despite variations in the voltage applied to the tube.

Another object is to provide such a circuit which is simple to construct and comprises inexpensive, readily available components.

A circuit constructed in accordance with the invention for stabilizing the gainof an electron multiplier phototube substantially at a constant level may comprise a voltage divider for connection across a voltage source.- The voltage divider may include a plurality of seriesconnected sections, two of which comprise means for maintaining a constant voltage thereacross with a third section located between the two constant voltage sections and preferably comprising means for adjusting the voltage thereacross. When the junctures of the sections of the voltage divider are connected to the multiplier phototube to maintain the electrodes thereof at successively increasing potentials, two stages of the tube will have constant voltages thereacross, which are substantially independent of variations in the power supply voltage, and a stage located between the constant voltage stages may have an adjustable voltage thereacross. If the output voltage of the power supply varies, the gain of the constant voltage stages changes in a direction opposite to the change in gain of the other stages, and thus compensates for the change in overall gain that would have resulted from the variation in power supply voltage. The inclusion of the adjustable voltage stage between the constant voltage stages permits the overall gain of the phototube to be maintained at one of various levels, and the gain may remain substantially constant at the predetermined level over a wide range of volts per stage applied to the phototube.

' Further objects and advantages of the invention will become apparent from the following description taken inconjunction with the accompanying drawing, in which Fig. 1 is a diagram of one form of the improved circuit of the invention; and

Fig. 2 is a diagram illustrating the gain characteristics of a typical electron multiplier phototube.

Referring now to the drawing, Fig. 1 illustrates a circuit arranged in accordance with the teaching of the invention for stabilizing the gain of an electron multiplier phototube 10 at substantially a constant level. The phototube 10 is representative of numerous commercially available types, and comprises a plurality of successively arranged electrodesincluding a photosensitive electronemitting cathode 11, a plurality of dynodes 12, and a collector or anode 13. In the particular case shown, the phototube 10' includes ten dynodes, which, along with the cathode and anode, constitute eleven stages of amplification. Potentials are supplied to the various electrodes of the multiplier phototube 10 from a high voltage supply (not shown), across which is connected a voltage divider designated? enerally by the numeral 14. The voltage divider 14 comprises a plurality of series-connected sections equal in number to the stages of amplification or.

tweenthe fourth andfifthdynodes and between the sixth The remainder" and seventh dynodes, the dynodes being taken in order of electron progression. Regardless of the number of stages of amplification of the phototube, it is preferred that the constant voltage stages are located one on each side of and adjacent the middle stage of amplification.

However, this arrangement is not to be construed as a limitation of the invention. One side of the high voltage supply and the cathode 11 of the phototube may be grounded as shown, and the anode 13 may be connected to the high voltage supply through a pair of output terminals 26. Of course, in order to have current flow through the anode 13, the circuit must be completed between the output terminals 26, as by connecting an ammeter, a resistor, or other suitable load therebetween. Thus, the successively arranged electrodes of the phototube are provided with successively increasing potentials when taken in the order of electron progression from cathode to anode.

The basic operation of the multiplier phototube is as described above in that electrons emitted from the photosensitive cathode 11 impinge on the first dynode and cause the emission of many other electrons, which are directed to the second dynode, where more new electrons are emitted. The multiplying process is repeated in each successive stage until the anode is reached, with the gain of each stage depending upon the potential difference between the two adjacent electrodes that define the stage. Assume, however, that the output voltage of the high voltage supply increases by a certain amount. In this case, the voltages across the variable resistor 17 and the fixed resistors 18-25, all of which are individually unregulated, will also increase, thus increasing the gain of the stages of the phototube that are connected across these resistors. However, the voltage across the stages that are connected across the glow discharge devices 15 and 16 remains constant, and the efiect of this action is to maintain the overall gain of the multiplier phototube substantially constant.

The theory of operation of the gain stabilizing circuit of the invention is not thoroughly understood at this time. However, it is believed that if the power supply voltage increases, thus increasing the voltage across the stages adjoining the constant voltage stages, defocusing of the electron beam occurs in the constant voltage stages and reduces their gain. This action compensates for the increase in gain of the other stages due to their increase in voltage. Conversely, if the power supply voltage decreases, the -focusing action increases in the constant voltage stages to increase their gain and compensate for the decreased gain of the other stages. In either case,

a the overall gain of the multiplier phototube remains substantially constant.

In order to adjust the circuit properly for various multiplier phototubes and to'provide means for maintaining the gain of the phototube at various levels, a variable resistor 17 may be provided between the glow discharge devices 15 and 16. The action of the variable resistor 17 is clearly seen in Fig. 2, which is a graph having volts per stage (potential diflference between adjacent electrodes of the tube) as the abscissa and log gain as the ordinate. The curve 27 shows the overall gain characteristics of a typical electron multiplier phototube, when connected across a conventional resistance voltage divider in the manner previously known. It is apparent from the steepness of the curve that a small change in volts per stage results in a great variation in the overall gain of the tube. The curves 28, 29, 30 and 31 represent the overall gain of the phototube when employed in the circuit of the invention with various values of the variable resistance 17, and the curves are numbered in order of increasing values of resistance. Thus, if a certain gain is required of the phototube, because of succeeding circuits that operate on the output of the phototube, the variable resistance 17 may be manually adjusted to set the gain of the phototube substantially at the desired level. -As the variable resistance '17 is adjusted to include more resistance in the circuit, the overall gain of the phototube is decreased until, as shown by the lowest curve 31, a setting of the variable resistance 17 may be reached at which the overall gain of the phototube is absolutely constant over a wide range of voltages per stage. As the amount of resistance is decreased, the gain of the phototube increases and the slope of the flat portions of gain curve becomes greater. In many instruments, however, the fact that the gain curve has some slope is of little importance, so long as the overall gain remains within certain specified limits. It is apparent, of course, that the variable resistance 17 may be replaced by a fixed resistance if the adjustable gain feature of the circuit is not desired.

The method of adjusting the variable resistance 17 is largely one of trial and error. Normally, the adjustment is made with the output of the high voltage supply at its average or desired level, and the variable resistance 17 is adjusted until the gain of the phototube is substantially at the level desired. Thus, the circuit might be adjusted so that the phototube is operating on the curve 30, and, with the high voltage output at its average value, the circuit might be operating at the point 30a. Thereafter, if the output of the high voltage supply varies, the operating point will vary along the curve 30 about the point 30a, and the overall gain of the phototube will remain substantially constant.

It is pointed out that the fixed resistors 18-25 of the voltage divider may be of equal value or may be of different values, depending on the phototube used and its method of-operation, and it is often desirable that the resistor 25 be made larger than the others when using certain types of phototubes. The constant voltage sections of the voltage divider 14 may comprise conventional neon glow discharge tubes, such as are available commercially from a number of sources, or they may comprise batteries or other sources of fixed voltage. Furthermore, the voltage divider need not be of the fixed resistance type, but may comprise variable resistances or other impedance elements. The circuit of the invention is not limited to the use of any particular type of phototube, nor is it limited to a phototube containing any particular number of stages of amplification therein.

The following table is presented as merely illustrative of values of circuit constants that might be used in a typical photomultiplier circuit constructed in accordance with the invention:

Electron multiplier phototube a RCA Type 5819. High voltage supply 1,000 volts. Glow discharge devices 15 and 16 GE Type NE 51. Variable resistance 17 1.5-3.0 megohms. Resistors 18-24 1.8 megohms. Resistor 25 4.7 megohms.

Although'I have shown and described a particular em bodiment of my invention, many modifications may be made and, therefore, I intend by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a circuit for an electron multiplier phototube'having at least three successively arranged electrodes con-' said third section being individually unregulated, and means for connecting said sections of said voltage divider to said phototube to provide successively increasing potentials to said successively arranged electrodes.

3. In a circuit for an electron multiplier phototube having a plurality of successively arranged electrodes, and a direct voltage source, the combination of a voltage divider for connection across said voltage source and comprising a plurality of sections, two of said sections comprising constant voltage means and having a third section comprising a resistance located between them, the remainder of said sections comprising resistances, the voltage drops across said third section and said remainder of said sections being individually unregulated, and means for connecting said sections of said voltage divider to said phototube to provide successively increasing potentials to said successively arranged electrodes.

4. In a circuit for an electron multiplier phototube having a plurality of successively arranged electrodes, and a direct voltage source, the combination of a voltage divider for connection across said voltage source and comprising a plurality of series-connected sections, two of said sections comprising glow discharge devices, a third section comprising a variable resistance and separating said glow discharge devices, the voltage drop across said third section being individually unregulated but manually adjustable, the remainder of said sections comprising fixed resistances, and means for connecting said sections of said voltage divider to said phototube to provide successively increasing potentials to said successively arranged electrodes.

5. In a circuit for an electron multiplier phototube having a plurality of successively arranged electrodes constituting a plurality of stages of amplification, and a direct voltage source, the combination of a voltage divider for connection across said voltage source and comprising a plurality of sections, two of said sections comprising constant voltage means and having a third section comprising a resistance located between them, the remainder of said sections comprising resistances, the voltage drops across said third section and said remainder of said sections being individually unregulated, and means for connecting said sections of said voltage divider to said phototube to provide successively increasing potentials to said successively arranged electrodes, said constant voltage sections being connected between those electrodes that define stages on each side of the center stage of amplification of the phototube.

6. In a circuit for an electron multiplier phototube having a plurality of successively arranged electrodes and a direct voltage source, the electrodes of the phototube including ten dynodes, the combination of a voltage divider for connection across said voltage source and comprising a plurality of series-connected sections, two of said sections comprising glow discharge devices, a third section comprising a variable resistance and separating said glow discharge devices, the remainder of said sections comprising fixed resistances with the voltage drops thereacross being individually unregulated, and means connecting said sections of said voltage divider to said phototube to provide successively increasing potentials to said successively arranged electrodes, one of said constant voltage sections being connected between the fourth and fifth dynodes and the other said constant voltage section being connected between the sixth and seventh dynodes, said dynodes being numbered in order of electron progression in the phototube.

References Cited in the file of this patent UNITED STATES PATENTS 

